Medical devices may contain drugs either to improve the tolerance, efficacy, or in vivo life-time of the device or use the device as a carrier for the drug. In either case the dose density (e.g. mg drug/mg device or mg drug/mm2 device surface), chemical stability, adherence, premature loss of drug, release rate, and total amount released are important and often critical features of the drug formulation. These properties are the more critical the more the requirements during production and application of the device vary or may even be contradictory. Drug-coated angioplasty catheters are typical examples: the drug coating must adhere firmly to tolerate mechanical stress during production including folding of balloons, crimping of stents, packaging, shipping to customers, and during final application, which involves passage through a narrow hemostatic valve, an introductory sheath or guiding catheter, and a variable distance of possibly tortuous and narrow blood vessels. When the balloon is inflated the drug should be released as rapidly and as completely as possible within a minute or less. The problem was demonstrated by Cremers et al. (Cremers B, Biedermann M, Mahnkopf D, Böhm M, Scheller B. Comparison of two different paclitaxel-coated balloon catheters in the porcine coronary restenosis model. Clin Res Cardiol 2009; 98:325-330), who retrieved as much as 50% of the dose from balloons after expansion for one minute in coronary arteries of pigs, whereas other catheters coated with the same drug and dose but in a different formulation released more than 95%. Almost perfect results (i.e., loss of only 10% of dose and only about 10% residual drug on the balloon after expansion in an artery) were achieved with a rigid prototype balloon (Scheller B, Speck U, Abramjuk C, Bernhardt U, Böhm M, Nickenig G. Paclitaxel balloon coating—a novel method for prevention and therapy of restenosis. Circulation 2004; 110: 810-814). The application of the same coating composition to more flexible modern balloon catheters resulted in problems, i.e., larger premature loss of the drug.
Another problem relates to the increased roughness of drug-coated medical devices, e.g., balloon catheters and stents. Usually, the surface is very smooth, which is accomplished by careful polishing in case of stents. In case of catheters a thin slippery hydrophilic layer may be coated on the surface to reduce friction inside guiding catheters and tortuous and narrowed vessel segments.